Polymeric materials which are insoluble at low pH but soluble in high pH are well known for enteric coating applications for medicaments. The polymers commonly used for enteric coatings are cellulose acetate phthalate (C-A-P), hydroxypropyl methylcellulose phthalate (HPMCP), cellulose acetate trimellitate (C-A-T), and hydroxypropyl methylcellulose acetate succinate (HPMCAS). Polyvinyl acetate phthalate (PVAP) and acrylic resins such as copoly(methacrylic acid/ethyl acrylate 1:1) and copoly(methacrylic acid/methyl methacrylate 1:1, or 1:2) are also known for their applications as enteric coating polymers. (See for example, Aqueous Polymeric Coatings for Pharmaceutical Dosage Forms, by J. W. McGinnity, Marcel Dekker, New York (1989), p. 81.)
In C-A-P for example, the specific pH at which it begins to dissolve is about pH 6.5. The specific pH is dependent on the degree of acetyl and phthalyl substitutions. The rate of dissolution of these polymeric enteric coating materials in alkaline media, although dependent on the nature of the buffer ions present in the solution, is relatively rapid.
A chapter entitled "Materials used in the film coating of oral dosage forms" by R. C. Rowe in the book edited by A. T. Florence and published by Blackwell Scientific Publications, p. 1, 1894, provides the art in general for formulating a pharmaceutical coating.
The polymers for enteric coating applications can be blended with each other to give a desirable dissolution profile at certain pH values, or blended with neutral water-soluble, or water-insoluble polymers for designing a drug delivery system. U.S. Pat. No. 4,795,641 teaches the composition of polymer blends containing a minor amount of cellulose acetate and a major amount of C-A-P, C-A-T, and cellulose acetate succinate. The blends have reverse phase morphology, that is, the minor component forms a continuous phase. The blends are useful for zero order controlled delivery of bioactive agents such as pharmaceutical agents and agricultural chemicals.
U.S. Pat. No. 3,489,743 describes a process for making cellulose mixed esters containing acetyl and phthalyl or hexahydrophthalyl moieties.
U.S. Pat. No. 4,960,814 describes water dispersible polymeric compositions based on polymers such as cellulose acetate phthalate.
Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Volume 5, pp. 118-143 (1979), John Wiley and Sons, New York, N.Y.; Encyclopedia of Polymer Science and Engineering, Volume 3, pp 158-181 (1985) John Wiley and Sons, New York, N.Y.; and Ullman's Encyclopedia of Industrial Chemistry, Fifth Completely Revised Edition, Volume A5, pp 438-447 (1986) VCH Verlagsgesellsehaft, Weinheim, Germany, provide general background information on cellulose ester technology.